Add a universal live datastream explorer view to the HTTP dashboard and point the e2e cluster at it by default.
- live_explorer: add the `LiveDatastreamExplorer` `DashboardView`, registered in `start_dashboard`, served at `/view/datastream/live` over the existing `/events` and `/api/frames` endpoints
- live_explorer_page: ship a 500-line self-contained page that auto-connects, renders streams/channels/frames metrics, and supports level drill-down (streams->channels->frames), payload-kind filtering, and per-channel retention limits
- local_e2e_cluster: switch the dashboard's default opened URL from `/view/mvp/cluster` to `/view/datastream/live`
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
